The present invention relates to an automatic power control device used for controlling the intensity of laser beam emitted by a laser emitting unit.
In a laser beam printer, a laser scanning device is provided for emitting laser beam to scan a photoconductive material to form a latent image. The laser scanning unit includes a laser diode which is driven to emit A laser beam carrying image information. The emitted laser beam is deflected by a mirror to scan a uniformly charged circumferential surface of a photoconductive drum to form a latent image thereon. Then the latent image is developed, and the developed image is transferred onto a recording sheet, and fixed thereon in accordance with an electro-photographic image forming process.
In the laser scanning unit, in order to maintain uniform intensity of the laser beam emitted by the laser diode, an APC (Automatic Power Control) device is provided.
The APC device has a photodiode which receives the laser beam emitted from the laser diode and outputs a signal having a current that corresponds to the intensity of the received laser beam. This outputted current produces a voltage across a resistor connected in series with the photodiode. This voltage is detected as the laser output voltage and also corresponds to the intensity of the laser beam. The laser output voltage is compared with a reference voltage, and then the current that flows through the laser diode is varied accordingly, so that the above-described output voltage is substantially the same as the fixed reference voltage.
In order to cancel the differences in characteristics of individual photodiodes employed in different devices, it is preferable that the resistance of the resistor connected with the photodiode be variable.
Conventionally, a variable resistor is connected to the photodiode, and its resistance is adjustable so that the difference in characteristics of the photodiodes, efficiencies of the optical systems and the like, are canceled.
However, even if a laser scanning unit can be adjusted to exactly emit the specified intensity of the laser beam, since there are differences in the characteristics amongst individual photoconductive drums, it is still necessary to further adjust the intensity of the laser beam. Further, if the resolution density of the image is varied, or the scanning speed of the laser beam is varied, it becomes necessary to change the intensity of the laser beam.
If the resistance of the resistor connected to the photodiode is made smaller than the required value, the laser output voltage becomes smaller even though the laser diode emits the same intensity of the laser beam. In this condition, if the APC operation is then executed, the intensity of the laser beam increases until the output voltage again reaches the reference voltage. Thus, one solution to solve the aforementioned problem is to change the resistance of the variable resistor. A change in the resistance of the variable resistor changes the voltage across it, provided that the laser beam intensity remains the same. However, since the variable resistor is used for canceling the differences in characteristics of the individual photodiodes, it is difficult to further adjust the resistance in order to control the intensity of the laser beam. Further, the above-described situation would usually occur after the APC device has already been installed in the laser scanning unit, and the laser scanning unit has been installed in the laser beam printer. After the device has been assembled, is difficult to change the resistance of the variable resistor.
Another way to vary the intensity of the laser beam is to provide a plurality of fixed resistors between a power source and the photodiode, and selecting one of them by means of a switch. Thus the intensity of the laser beam can easily be changed by operating the switch. However, in this example, the number of intensity levels are determined by the number of resistors. In order to obtain a large number of intensity levels, a large number of resistors are required to be connected, resulting in complicated circuitry. Further, the intensity selected is a predetermined value, and therefore subtle changes or changes to values between the predetermined values cannot be made with this construction.